1. Technical Field
The present invention relates generally to a camera and a method in a camera. In particular it relates to the determination of camera settings when using pulsed flash.
2. Description of Related Art
Camera phones usually have a white Light Emitting Diode (LED) for illumination of an object in dark environments. The light output from an LED is very low which makes it necessary to use long exposure times. Long exposure time is a problem as hand shake of the user results in blurred images. Recent technical progress has made more powerful LEDs available. These LEDs can produce a good amount of light even at short pulses. It is also possible to reach higher light levels with short pulses as power dissipation limits the efficiency of an LED
The brightness of an object depends on both ambient light and the light coming from the LED. The effect of the LED varies depending on the distance to the object and the reflectivity of the object. Hence it is difficult to adjust the camera for correct exposure as these conditions are normally not known.
The camera automatically adjusts the exposure settings when a continuous lighting LED is used. This is possible since the camera can capture several image frames when the LED is lit and adjust accordingly. This method is not possible to use with a pulsed LED as the brightness of the object will come as a total surprise.
One way to solve this problem is to regulate the LED power the same way a Xenon flash is regulated. A light sensor integrates the reflected light from the object and an electronic circuit turns off the flash when a predefined light flux is reached.
There are however some disadvantages with this solution. The predefined flux must be set at a level low enough to ensure enough flux at the maximum desired range of the device. The effect will be that we only take full advantage of the LEDs capabilities when it is used at its maximum working range. This would be to waste performance in most cases.
For example, if the maximum working range is set to 2 meters, only 25% of the LEDs potential will be used at 1 meter.
Yet another problem has to do with white balancing. A typical indoor environment is illuminated by tungsten bulbs having a colour temperature of about 2800 K (very reddish compared to daylight). A typical white LED has a colour temperature of about 7000 K. White balancing becomes very difficult as we don't know to what extent the LED contributes to the total illuminating of the scene.
Ideally the LED should always be used at maximum power and the camera exposure setting (gain and exposure time) should be adjusted to match the circumstances.